DSP / Active Filter Types / Slopes - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 5 Old 05-21-15, 10:59 PM Thread Starter
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DSP / Active Filter Types / Slopes

Is there any kind of discussion out there on filter types and slopes, advantages / disadvantages? I recall from my passive crossover days that certain filter combinations had certain characteristics, and I assume the same runs true for active dsp crossovers, but I'd be interested in a good discussion of these characteristics.
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post #2 of 5 Old 05-22-15, 07:40 AM
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Re: DSP / Active Filter Types / Slopes

It's big and important area of speaker design. It's not easy to address without some info on your situation and area of interest.

There are lots of good options that provide excellent result results. I am well experienced with minimum phase XO approaches, having evaluated several hundred different options on my 5.2 speaker setup (12 DCX channels). I do implement an FIR phase correction filter in my music server to remove the phase rotation to achieve a linear phase condition for music listening. I honestly can't tell any difference using the FIR filter, but of course the GD and step responses look a lot better that way and others with better hearing may find a difference.

More attention is now given to FIR linear phase XOs as now the option has become much easier to implement. It's possible to get smoother looking graphs that way and many suggest that takes sound quality to an even better level. With no experience yet, it is an open issue for me. I must say I am skeptical as to how much room there is for improvement however.

I wouldn't expect either method to be significantly better when it is well implemented as an overall system. The choice is more to do with the equipment capabilities you want to put in place and which method you find more appealing to deal with. I don't think either is easy to optimize, but now there is linear phase software that makes that more attractive to someone following that direction.

I wanted to investigate all the options using the minimum phase choices as a learning experience to see for myself which of the options worked best for me. There are lots of different camps that tout; "this filter is better than that one" or "this method works better than that".

That's the big picture for me. Within each approach there are lots of choices and tradeoffs to consider. If you have questions regarding the minimum phase approach, I can comment. Given some of your background info, I can offer my opinion on direction. If you are interested, I can also provide lots of graphs showing overall results possible and detailing key characteristics to consider.
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post #3 of 5 Old 05-23-15, 08:19 PM Thread Starter
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Re: DSP / Active Filter Types / Slopes

jtalden wrote: View Post
It's big and important area of speaker design.<snip>If you are interested, I can also provide lots of graphs showing overall results possible and detailing key characteristics to consider.

My setup is using minidsp's for crossovers & equalization. I'm not really looking for specifics on how to setup my system, but more a general discussion regarding the different combinations of crossover type (LR, BW etc) and slope and the characteristics each has. Primarily I'm looking for information regarding phase characteristics, and to that end I'd be interested in how you went about implementing the FIR filters. I don't think it's an option with the minidsp's I have (2x4 and 4x10) but still interesting.

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post #4 of 5 Old 05-23-15, 10:12 PM
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Re: DSP / Active Filter Types / Slopes

What equipment are you guys using for FIR filters in home audio? Lab Gruppen has been my only experience with them.


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post #5 of 5 Old 05-24-15, 09:39 AM
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Re: DSP / Active Filter Types / Slopes

For the FIR filter implementation I use FooBar2000 media server with the DLNA and Convolve plugins. The filter itself is created using rePhase.exe. This is a freeware solution. If you don't stream music from a computer then some other hardware device is needed. As you noted the DCXs and the 2x2/4x10 can't do it. I can't hear any definitive difference even through headphones. I only do this because I can in my setup and maybe others with better hearing can spot differences.

Regarding IIR filter types and slopes for XOs; it is difficult to separate the filter choices from drivers used. Comparisons of different filter setups is very iffy because the choices of house curve and EQ also enter into the setup. It becomes impossible in my case to clearly associate an improvement to only the filter choice. Very good results are possible with various choices. There are various tradeoffs and it's difficult to generalize.

I can offer some general thoughts:
> Close phase tracking of the direct sound to the LP throughout the entire XO is a good objective.
> For protection of most common drivers, it's best to use slopes of 12dB or more.
> It's difficult to implement a 48dB acoustic XO that phase tracks closely with typical DSP box settings.
> Choose an XO frequency that considers the horizontal coverage angle of the 2 drivers.
> The phase differences between the filter types is trivial so choose the filter type based on its impact to SPL.
> SW/Main XOs can be very difficult to evaluate as the room modes and reflections can make it very difficult to find the best solution.

For my current 2k XO I found the best results to be settings that approach the popular acoustic LR-24. The primary emphasis here was choosing filters that provide very close phase tracking throughout the entire XO range. Because of driver characteristics the filters needed were as follows.
> LPF, Bessel-24, 1.82 kHz
> HPF, Butterworth-18, 2.25 kHz

For flattest SPL there would have been even more spread of the filters, but the close phase tracking was lost if that was done. The resulting SPL peak (not shown) was tamed using appropriate EQ on the DCX input to impact both drivers identically.

Following are near-field results on listening axis (all at 1/24 octave smoothing)

Below shows XO range is from about 500-8k for about -50dB SPL suppression:
DSP / Active Filter Types / Slopes-spl-range-1.png

Below shows the close phase tracking achieved with these settings:
DSP / Active Filter Types / Slopes-phase-tracking-2.png

Below shows the phase tracking with the FIR filter active:
DSP / Active Filter Types / Slopes-phase-using-rephase-fir-filter-3.png

This is just an example; not the only way to get good results. I do judge this to be the best setup I have found to allow strong clear highs without any trace of voice harshness. The detail and stage image is also very good.
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active , dsp , filter , slopes , types

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